I know this will be hard to answer but a 42' Roughwater with 8.2 Detroits at 250hp each, approximately how much diesel should it burn at 8 knots?
And will running these at that low speed be harmful? Not sure what the rpms would be?
The owner claims at 10 knots they turn 2000 rpms but doesn't know the fuel burn. He also doesn't know what the hours are (he's trying to sell it for his dad). I know maintenance and how they were run matters a lot but how many hours should these go with typical maintenance?

I would add into your research the availability of parts as the 8.2's are pretty out of date and those four strokes were not the most popular as apposed to the 2 stroke ones that were put in everything, that being said at 250hp and low load would out last anything you most likely will be doing.
Fuel burn would be pretty low as those motors were pretty good, total guess would be 6-8gph.

Back in the late '80's my father had a Luhrs 34 with the J&T 8.2L TI engines at 300hp IIRC. He ran them at a reasonable RPM, being a bit of a conservative man, and we never had any engine issues in a bout 1600 hrs or so until he sold it. I did have a lot of time in the 'hole' changing out Racor filters in nasty conditions but if fed clean fuel and not overheated they were reliable. As noted, parts may be a concern but if cruising in coastal US waters probably not a significant issue since the engine was also used in road vehicles as well.

A very rough rule of thumb is 2 hp per thousand pounds to push a semi displacement boat to hull speed. So assuming 40,000 lbs that is 80 hp for 8+ kts. That engine probably makes about 15 hp per gph at 40 hp each, so your total fuel burn should be about 80/15= 5 gph. But could be 50% more. 5-8 gph would be the range I would work with.

I think originally they had cooling issues, but I think that was corrected. After that, it was pushed to 300+ HP. We all know how that works. When not pushed hard, I think the service life would be very good.

I believe it was deserved. They went from 13mm to 14mm head bolts to solve some head issues.
That engine was designed for services in busses and they blew up pretty quickly. The constant speed , constant load service in boats it faired much better. The Detroit Diesel dealers and mechanics didn't have much good to say about them once the problems started. Not very well supported by DD.

We have 250hp 8.2's in our Atlantic and i have been extremely happy with the the fuel burn. I calculated the last day of our recent 4 day cruise at about 4.5 GPH, 8 knots at 1800 RPM. At just over 1700 hours now and we have done 200 of that since last year when we purchased at all about that same rate, plan to keep going with them as long as we can.

Had similar concerns and did all the research before purchase and agree with several comments. The loser engines were the 350 hp that had over heat problems and earlier models with smaller head bolts. Many mechanics i spoke to really like the engines provided they had not been overheated - my real concern is that there are very few folks that will actually work on them. Our local DD diesel won't but I did find a local mechanic - thankfully haven't needed anything yet (knocking on wood...)

I have had two myself and they are as smooth as a car engine compared to what I have endured with Cummins - which I can't stand to listen to anymore. DD 8.2's sing like an industrial bench engine at higher RPM, absolutely clear exhaust as well with 2000 hours. But yes, 250 HP, not something like a 375 hp 3208, which has a service life of 30,000 gallons - quite modest compared to a reliable bus engine like the 8.2.

I have had two myself and they are as smooth as a car engine compared to what I have endured with Cummins - which I can't stand to listen to anymore. DD 8.2's sing like an industrial bench engine at higher RPM, absolutely clear exhaust as well with 2000 hours. But yes, 250 HP, not something like a 375 hp 3208, which has a service life of 30,000 gallons - quite modest compared to a reliable bus engine like the 8.2.

????

A straight 6 Cummins is about as smooth as it gets!!! But....just an opinion!

Ski - when i first posted about our 8.2's your comment mentioned a road load governor on these engines, i never have figured out what that is? not an issue when cruising at trawler speeds?

A "road governor" mimics how a gas engine behaves with a gas pedal: give it a bit of pedal and it will rev high. A normal diesel governor is a "variable rpm" where no matter where you set the throttle, rpm will stay within say 10% of the setting. The road governor, or "speed limiting" governor will rev way up in neutral if you just crack the throttles. Anyone that owns 8.2's will understand the difference.